1. Field of the Invention
The present invention relates to a method of producing a semiconductor device such as a thin film transistor (TFT), and more particularly to a method of producing a semiconductor device using a crystalline silicon film on a substrate having an insulating surface.
2. Description of the Related Art
In recent years, attention has been paid to a technique by which a thin film transistor (TFT) is formed using a silicon thin film formed on a glass substrate. The TFT is primarily used for a liquid crystal electro-optical device of the active matrix type. Also, the TFT is used for a variety of thin film integrated circuits.
The liquid crystal electro-optical device is designed such that liquid crystal is sealingly interposed between a pair of glass substrates, and the optical characteristic of the liquid crystal is changed by the application of an electric field to the liquid crystal, to display an image.
The liquid crystal display unit of the active matrix type for which the TFT is used is characterized in that a TFT is arranged in each pixel, and the charges stored by a pixel electrode are controlled by the TFT that functions as a switch. The liquid crystal display unit of the active matrix type is used as a display for various electronic devices (for example, a portable word processor or a portable computer) because it is capable of displaying a fine image at high speed.
The TFT used in the liquid crystal display unit of the active matrix type is generally produced on a transparent substrate such as a quartz substrate or a glass substrate and uses an amorphous silicon thin film. However, the TFT using the amorphous silicon thin film has problems stated below.
(1) The characteristic is low, and image display with higher quality cannot be performed.
(2) A peripheral circuit for driving a TFT disposed in each pixel cannot be constructed.
As a method for solving the problems (1) and (2), there is a technique for producing the TFT using a crystalline silicon thin film. As a method for obtaining the crystalline silicon thin film, after the amorphous silicon film is formed on a glass substrate or a quartz substrate by plasma CVD or low pressure CVD, there is a method that an amorphous silicon film is thermally treated, and a method that a laser beam is irradiated onto an amorphous silicon film.
The problem (2) is considered to be classified into such a problem that a CMOS circuit cannot be constructed since the TFT using the amorphous silicon thin film does not put a p-channel type TFT into practical use, and such a problem that the peripheral drive circuit cannot be assembled since the TFT using the amorphous silicon thin film cannot operate at high speed and also a large current cannot flow therein.
As a method in which the amorphous silicon film is crystallized by a heat treatment, there has been known a structure disclosed in Japanese Patent Unexamined Publication No. 6-232069. This method enables a crystalline silicon film to be obtained under a condition in which a heat treatment is conducted at 550xc2x0 C. for 4 hours, using a metal element such as nickel which promotes the crystallization of silicon. Hence, the crystalline silicon film can be formed even on a glass substrate lower in heat resistance than a quartz substrate.
However, according to the above publication, there arises an unsatisfactory matter in the crystallinity of a crystalline silicon film to be obtained. In other words, the crystallinity of the crystalline silicon film to be obtained is low, which causes a large amount of amorphous silicon components to remain.
Also, if the heat treatment conditions are 550xc2x0 C. and 4 hours, the crystalline silicon film can be formed at a level where no problem is caused by distortion or deformation of a glass substrate of about 10 inches in size, using a metal element such as nickel. However, there is a demand that the liquid crystal display unit is increased in area, and it is expected that a liquid crystal display device of 20 inches, further 30 inches or more diagonally will be produced. In such a large area, the distortion or deformation of the glass substrate causes a problem even by the technique disclosed in the Japanese Patent Unexamined Publication No. 6-232069 is used.
The liquid crystal electro-optical device is so designed as to interpose a liquid crystal between a pair of glass substrates with an interval of several xcexcm. Thus, the distortion of xcexcm order between the edges of the substrate causes the thickness of a liquid crystal layer to be changed. This causes the nonuniformity of display and so on. Also, in producing a semiconductor integrated circuit formed on a glass substrate, the problem leads to the lowering of a yield accompanied by the failure of exposure or the failure of substrate transportation.
In the Japanese Patent Unexamined Publication No. 6-232069, there is observed such a phenomenon that a metal element such as nickel as used is locally concentrated in the crystalline silicon film. This phenomenon comes to a factor that leads to a defect when constituting the device. Also, this becomes a factor that lowers the yield of production.
Further, as a technique in which an amorphous silicon film is transformed into a crystalline silicon film, there has been known a technique in which a laser beam is irradiated onto the amorphous silicon film. The irradiation of a laser beam enables the crystalline silicon film partially high in crystallinity to be obtained. However, it is difficult to obtain the effect caused by the irradiation of a laser beam with high reproducibility, and also it is difficult to obtain the crystalline silicon film over a large area.
An object of the present invention disclosed in the specification is to provide a technique in which a crystalline silicon film excellent in crystallinity is formed on an insulating surface of a glass substrate, a quartz substrate or the like, and particularly to provide a technique in which a crystalline silicon film with high crystallinity is formed on the glass substrate in a state where the flatness of the glass substrate is maintained.
Another object of the present invention is to solve the above problems, and to provide a method of producing a semiconductor, so as to obtain a crystalline silicon film in which a metal element is not locally concentrated in the case where an amorphous silicon film is crystallized using the metal element that promotes the crystallization of silicon.
To solve the above problems, according to the present invention, there is provided a method of producing a semiconductor device including the steps of: disposing metal elements that promote the crystallization of silicon in contact with the surface of an amorphous silicon film formed on an insulating surface; and subjecting the amorphous silicon film to a thermal processing at a temperature of the crystallization temperature or higher of the amorphous silicon film to crystallize the amorphous silicon film.
In the above structure, the substrate having the insulating surface may be formed of a glass substrate, a glass substrate on which an insulating film is formed, or a semiconductor substrate on which an insulating film is formed.
The amorphous silicon film may be formed by plasma CVD or low pressure thermal CVD. The amorphous silicon film formed by low pressure thermal CVD is preferable because hydrogen contained therein is little, and it can be crystallized easily.
The metal element that promotes the crystallization of silicon may be one kind or plural kinds of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. The use of Ni (nickel) in the metal elements is preferable because its effect is high and the reproducibility is high.
The crystallization temperature of the amorphous silicon film depends on a method or condition of forming the silicon film. Since crystallization is made by a long heating time even at a low temperature, there is no definite boundary of a temperature at which the crystallization starts. Even the amorphous silicon film which is crystallized by heating at 600xc2x0 C. for 24 hours can be transformed into a complete crystalline silicon film by a heat treatment at 580xc2x0 C. for 96 hours.
In the above structure, a temperature at which the overall amorphous silicon film is crystallized in the heat treatment for 12 hours is defined as a crystallization temperature. In this specification, that the overall amorphous silicon film is crystallized is directed to a state in which 80% or more of the overall amorphous silicon film is transformed into a crystalline component. Also, the state in which the overall amorphous silicon film is crystallized may be a state in which the spectrum of the amorphous component hardly appears through the measurement of Raman spectroscopy, and the spectrum of the crystalline component becomes remarkable.
According to the present invention, there is provided a method of producing a semiconductor device including the steps of disposing metal elements that promote crystallization of silicon in contact with the surface of an amorphous silicon film formed on an insulating surface; subjecting the amorphous silicon film to a thermal processing at a temperature which is a strain point or higher of the glass substrate and an annealing point or lower of the glass substrate to crystallize the amorphous silicon film; and cooling the crystallized silicon film at 1xc2x0 C./min or lower until a 500xc2x0 C. or lower after the thermal processing.
In the above structure, the step of thermally processing the amorphous silicon film at a temperature which is a strain point or higher of the glass substrate, and an annealing point or lower of the glass substrate, to crystallize the amorphous silicon film is because a stress in the glass substrate is released so as to prevent the glass substrate from being strained, deformed or contracted later. It is not preferable to conduct the heat treatment at the strain point or lower of the glass substrate because its effect is small. Also, it is not preferable to conduct the heat treatment at the annealing point or higher. This is because the generation of a new stress is observed. The annealing point is a temperature at which the viscosity is 4xc3x971014 poise and an upper limit temperature when removing the distortion in glass. In this temperature, the stress in glass disappears after several to ten several minutes because of the viscous flow.
Also, the reason why cooling is conducted at 1xc2x0 C./minute or lower is to prevent the substrate from being distorted or deformed by a stress generated by cooling. The cooling temperature is preferably as low as possible for the purpose of releasing the stress. However, the low cooling temperature leads to such an economical problem that a processing time becomes long. The cooling is necessary to conduct up to 500xc2x0 C. or lower. However, the cooling of even a room temperature is not economical because it takes time.
The heating and cooling are effective when they are conducted on a base object (substrate) that a flatness is ensured. This is because the glass substrate is largely affected by the state of the surface of the base object on which the glass substrate is located when heating is conducted at a temperature which is the strain point or higher of the glass substrate and the annealing point or lower. In the case where the glass substrate is located on the curved base object and the heating treatment is conducted, the glass substrate is deformed into a shape which is along the shape of the curved base object. Conversely, when the heat treatment is conducted using the base object that the flatness is ensured, the glass substrate is formed into a shape which is along the flatness of the base object, to obtain the glass substrate excellent in flatness.
Since the above fact is also applied to a case when cooling starts, it is important to dispose the glass substrate on the object that the flatness is ensured even in the cooling process.
According to the present invention, there is provided a method of producing a semiconductor device including the steps of: disposing metal elements that promote the crystallization of silicon in contact with the surface of an amorphous silicon film formed on a glass substrate; subjecting the amorphous silicon film to a thermal processing at a temperature which is a crystallization temperature or higher of the amorphous silicon film, a strain point or higher of the glass substrate and an annealing point or lower of the glass substrate to crystallize the amorphous silicon film; and cooling the crystallized silicon film at 1xc2x0 C./min or lower until 500xc2x0 C. or lower after the thermal processing.
In the method of producing the semiconductor device having the above steps, in the state where the metal elements that promote the crystallization of silicon are disposed in contact with the surface of the amorphous silicon film, the amorphous silicon film is thermally processed at a temperature which is the crystallization temperature or higher of the amorphous silicon film, thereby obtaining a crystalline silicon film having a high crystallinity. The crystalline silicon film is higher in crystallinity than a crystalline silicon film obtained by simply giving heating to the amorphous silicon film as well as a crystalline silicon film obtained by heating the amorphous silicon film at a temperature which is the crystallization temperature or less of the amorphous silicon film in a state where the metal elements that promote the crystallization of silicon are disposed in contact with the surface of the amorphous silicon film.
The metal elements that promote the crystallization of silicon are disposed in contact with the surface of the amorphous silicon film formed on the glass substrate, and the heating treatment is conducted on the amorphous silicon film at a temperature which is the strain point or higher of the glass substrate and the annealing point or less of the glass substrate, and further a crystallized silicon film is cooled at 1xc2x0 C./min or lower until 500xc2x0 C. or lower, thereby obtaining a crystalline silicon film in a state where the stress in the glass substrate is released. In particular, in the heating and cooling processes, the glass substrate is disposed on the substrate that the flatness is ensured, thereby obtaining the crystalline silicon film on the glass substrate that the flatness is ensured.
The metal elements that promote the crystallization of silicon are disposed in contact with the amorphous silicon film formed on the glass substrate, and the heating treatment is conducted on the amorphous silicon film at a temperature which is a crystallization temperature or higher of the amorphous silicon film, a strain point or higher of the glass substrate and an annealing point or lower of the glass substrate, thereby obtaining a crystalline silicon film having a high crystallinity on the glass substrate where inside stress is released. In particular, in the heating process and the subsequent cooling process, the glass substrate is disposed on the object that the flatness is ensured, thereby obtaining the crystalline silicon film on the glass substrate that the flatness is ensured.
According to the present invention, there is provided a method of producing a semiconductor device including the steps of: disposing metal elements that promote the crystallization of silicon in contact with the surface of a silicon film which is formed on a quartz substrate; and thermally processing the silicon film at 800 to 1100xc2x0 C. to transform the silicon film into a crystalline silicon film or promote the crystallinity of the silicon film.
According to the present invention, there is provided a method of producing a semiconductor device including the steps of: applying a solution containing metal elements that promote the crystallization of silicon on a silicon film formed on a quartz substrate; and thermally processing the silicon film at 800 to 1100xc2x0 C. to transform the silicon film into a crystalline silicon film or promote the crystallinity of the silicon film.
According to the present invention, there is provided a method of producing a semiconductor device including the steps of: disposing metal elements that promote the crystallization of silicon in contact with the surface of an amorphous silicon film formed on a quartz substrate; and thermally processing the amorphous silicon film at 200xc2x0 C. higher than a crystallization temperature of the amorphous silicon film to transform the amorphous silicon film into a crystalline silicon film.
According to the present invention, there is provided a method of producing a semiconductor device including the steps of: patterning an amorphous silicon film formed on a quartz substrate to form an island region having a diameter of 200 xcexcm or less; disposing metal elements that promote the crystallization of silicon in contact with the surface of the island region; and thermally processing the island region at 800 to 1100xc2x0 C. to crystallize the island region.
The substrate may be formed of a semiconductor substrate represented by a single-crystal silicon wafer instead of the quartz substrate. In the semiconductor substrate, there arise such a problem that the transmittivity of light cannot be ensured, and such a problem that it is necessary to form an insulating film on the surface of the semiconductor substrate.
In the present invention, a single layer film selected from a silicon oxide film, a silicon nitride film and a silicon nitride oxide film, or a multilayer film made up of those films, which is formed on the quartz substrate, is also called xe2x80x9csubstratexe2x80x9d. In general, in order to release a stress generated between the quartz substrate and the semiconductor film, it is preferable to form an under film such as a silicon oxide film.
Also, the present invention can be applied to a technique in which an insulating film is formed on an integrated circuit using a silicon wafer (in general, called as xe2x80x9cICxe2x80x9d), and a TFT is formed on the insulating film as an under film. That is, the silicon wafer (or a base object made of single-crystal silicon) on which the integrated circuit is formed as required for a substrate can be used as a base object.
The silicon film may be formed of an amorphous silicon film or a microcrystal silicon film. In particular, it is effective to use an amorphous silicon film, in which the contents of hydrogen is reduced as much as possible. Also, in order to artificially reduce the hydrogen in the amorphous silicon film, it is very effective to thermally process the amorphous silicon film at 300 to 500xc2x0 C. for about 30 minutes to 2 hours to promote the elimination of hydrogen from the film. A heating treatment for crystallization may be conducted after the thermal processing for elimination of hydrogen.
In the method of producing the semiconductor device having the above steps in accordance with the present invention, the temperature of the heat treatment for crystallizing the silicon film formed on the quartz substrate or the silicon wafer or improving its crystallinity is preferably 800 to 1100xc2x0 C. Also, in case of using the amorphous silicon film as a starting film, the temperature of the heat treatment is preferably set to a temperature which is higher than the crystallization temperature of the amorphous silicon by 200xc2x0 C. or higher.
The crystallization temperature of the amorphous silicon film is different depending on the method or conditions for forming the silicon film. Since the crystallization is conducted on the amorphous silicon film by a long heat time even at a low temperature, there is no definite boundary of a temperature at which the crystallization starts. Even the amorphous silicon film which is barely crystallized by heating at 600xc2x0 C. for 24 hours can be transformed into a complete crystalline silicon film by a heat treatment at 590xc2x0 C. for 96 hours.
In the present invention, a temperature at which the overall amorphous silicon film is crystallized in the heat treatment for 12 hours is defined as a crystallization temperature. In this specification, that the overall amorphous silicon film is crystallized is directed to a state in which 80% or more of the overall amorphous silicon film is transformed into a crystalline component. Also, the state in which the overall amorphous silicon film is crystallized may be a state in which the spectrum of the amorphous component hardly appears through the measurement of Raman spectroscopy, and the spectrum of the crystalline component becomes remarkable.
The crystallization temperature of the amorphous silicon film is generally 580 to 620xc2x0 C., although it depends on the film forming method or conditions.
The metal elements that promote the crystallization of silicon is used, and the heat treatment for obtaining the crystalline silicon film on the quartz substrate is conducted at a high temperature of 800 to 1100xc2x0 C., thereby obtaining the crystalline silicon film having a high crystallinity by the heat treatment for a short time. Also, such a heat treatment at a high temperature is conducted, thereby preventing the metal elements from being locally concentrated in the silicon film.
According to the present invention, there is provided a method of producing a semiconductor device including the steps of: intentionally introducing metal elements that promote the crystallization of silicon into an amorphous silicon film to crystallize the amorphous silicon film by a first heat treatment; conducting a second heat treatment on the amorphous silicon film; and forming a silicon nitride oxide film on the silicon film; wherein the second heat treatment is conducted at a temperature which is the same as or higher than that of the first heat treatment.
In the above method, the second heat treatment is conducted at 550 to 1050xc2x0 C., more preferably, at 600 to 980xc2x0 C.
A laser light is irradiated after the second heat treatment, to form a single-crystal region or a substantially single-crystal-like region where no grain boundary substantially exists, with a spin density of 3xc3x971017 cmxe2x88x923 or less.
In the method of producing the semiconductor device having the above steps according to the present invention, after the heat treatment for the crystallization, another heat treatment is conducted at a temperature higher than that of the heat treatment for crystallization, thereby preventing the metal intentionally introduced into the film for promoting the crystallization of silicon from being locally concentrated.
In the present invention, a method of applying a solution containing the metal elements therein on the surface of the amorphous silicon film is optimum as the method of disposing the metal elements that promote crystallization of silicon in contact with the amorphous silicon film.
Using that method, the concentration of the metal element in the solution is adjusted so that the concentration of the metal element finally existing in the silicon film can be adjusted. The concentration of the metal element existing in the silicon film is necessarily set to the concentration of about 1xc3x971015 to 5xc3x971019 atoms cmxe2x88x923, preferably 1xc3x971016 to 5xc3x971017 atoms cmxe2x88x923. Thus, the above method using the solution is very useful. The concentration of the metal element is defined as a minimum value measured by SIMS (secondary ion mass spectrometry).
It has been found that the method using the solution permits the metal element to be disposed in uniform contact with the surface of the amorphous silicon film. This means that a layer of the metal element or a layer containing the metal element therein can exist in uniform contact with the amorphous silicon film. This becomes very important because it prevents the metal elements from being locally concentrated.
In case of using nickel as a metal element, a solution can be used which mainly contains at least one kind of nickel compound selected from nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel acetylacenate, nickel 4-cyclohexyl butyrate, nickel oxide, nickel hydroxide, and nickel 2-ethyl hexanoic acid.
Also, Ni may be mixed with at least one selected from benzene, toluene, xylene, carbon tetrachloride, chloroform, ether, trichloroethylene, or freon, all of which are nonpolar solvents.
In case of using Fe (iron) as a metal element, its compound known as ion salt, for example, at least one kind of material selected from iron (I) bromide (FeBr26H2O), iron (II) bromide (FeBr36H2O), iron (II) acetate (Fe(C2H3O2)3xH2O), iron (I) chloride (FeCl24H2O), iron (II) chloride (FeCl36H2O), iron (II) fluoride (FeF33H2O), iron (II) nitrate (Fe(NO3)39H2O), iron (I) phosphorate (Fe3(PO4)28H2O), and iron (II) phosphorate (FePO42H2O) can be used as a main component.
In case of using Co (cobalt) as a metal element, its compound known as a cobalt salt, for example, a material selected from cobalt bromide (CoBr 6H2O), cobalt acetate (Co (C2H3O2)24H2O), cobalt chloride (CoCl26H2O), cobalt fluoride (CoF2XH2O), and cobalt nitrate (Co(No3)26H2O) can be used as a main component.
In case of using Ru (ruthenium) as a metal element, its compound known as ruthenium salt, for example, ruthenium chloride (RuCl3H2O) can be used.
In case of using Rh (rhodium) as a metal element, its compound known as a rhodium salt, for example, rhodium chloride (RhCl33H2O) can be used.
In case of using Pd (palladium) as a metal element, its compound known as a palladium salt, for example, palladium chloride (PdCl22H2O) can be used.
In case of using Os (osmium) as a metal element, its compound known as osmium salt, for example, osmium chloride (OsCl3) can be used.
In case of using Ir (iridium) as a metal element, its compound known as iridium salt, for example, a material selected from iridium trichloride (IrCl33H2O) and iridium tetrachloride (IrCl4) can be used as a main compound.
In case of using Pt (platinum) as a metal element, its compound known as platinum salt, for example, platinum (II) chloride (PtCl45H2O) can be used.
In case of using Cu (copper) as a metal element, its compound, a material selected from copper (II) acetate (Cu (CH3COO)2), copper (II) chloride (CuCl22H2O) and copper (II) nitrate (Cu(NO3)23H2O) can be used.
In case of using gold as a metal element, as its compound, a material selected from gold trichloride (AuCl3xH2O) and gold nitride (AuHCl44H2O) can be used.
In order to adjust the concentration of those metal elements, it is effective to dilute the above materials with an appropriate solvent. Also, it is effective to contain a surface active agent in the above solvent. The use of a surface active agent permits the metal elements to be dispersed on the surface of amorphous silicon film, thereby enhancing the effect that allows the metal elements to exist.